JPEG, a lossy image compression standard based on the discrete cosine transform (DCT), was introduced in 1992. JPEG is firmly entrenched in digital cameras (e.g., Nikon™), smart phones (e.g., Android RAZR™), and web browsers (e.g., Internet Explorer™, Firefox™, etc.). Additionally, JPEG is the basis of today's MPEG-2 high definition television (HDTV) digital satellite news gathering (DSNG) broadcast industry and H.262 video compression.
JPEG-2000 (i.e., JP2), also a lossy image compression standard, but based on the discrete wavelet transform (DWT), was published in 1999. JPEG-2000 has been a JPEG-competing standard since its inception. In 2005, the U.S. Library of Congress embraced JPEG-2000 for permanent electronic storage of digitized images. JPEG-2000 is also supported by other smart phone brands (e.g., Apple iPhone™).
The estimated spectrum adaptive postfilter (ESAP) reuses the DCT coefficients of a decoded grayscale JPEG image to postfilter and enhance the image. ESAP pixel-adaptive filters minimize the JPEG 8×8 blocking distortion without requiring any additional bit stream overhead. A key improvement is that ESAP not only adaptively filters out the JPEG blocking distortion, but also does not introduce the ringing artifacts inherent to JPEG-2000. ESAP complies with the Independent JPEG Group (IJG) Software™, Release 8c, found at http://www.iig.org. Baseline JPEG uses Huffman entropy coding, whereas JPEG-2000 and ESAP-AC both use arithmetic entropy coding (AC).
Most of the quality improvement of JPEG-2000 over baseline JPEG images is likely due to the arithmetic entropy coding utilized by JPEG-2000 versus the Huffman coding initially used by JPEG. Similarly, when JPEG uses AC instead of Hoffman coding, as in the IJG Software™ command example:                cjpeg -quality 5 -arithmetic img.ppm img.jpg,        
a remarkable visual improvement of the JPEG image is observed. When the image is further filtered by the pixel-adaptive ESAP process, the ESAP-AC image visual and peak signal-to-noise ratio (PSNR) quality either matches or surpasses the quality of JPEG-2000. The excellent DCT energy compaction properties are thus exulted with usage of AC.
A previous version of the ESAP algorithm was applied to grayscale images only. ESAP, in its most recent version, operates in the RGB color image space.